In boreal winter, gale-force wind gusts, widespread heavy precipitation, and cold-air outbreaks constitute some of the most severe weather hazards affecting Europe. Advancing our understanding of the synoptic- to micro-scale processes and their representation in numerical weather prediction (NWP) models leading to such hazardous high impact weather (HIW) is the overarching aim of the ground-based and airborne “North Atlantic Waveguide, Dry Intrusion, and Downstream Impact Campaign” (NAWDIC, https://www.nawdic.kit.edu/). The core element of the international NAWDIC consortium is NAWDIC-HALO. The NAWDIC-HALO consortium comprises German institutions working on topics of mid-latitude atmospheric dynamics. It is led by the Institute of Meteorology and Climate Research, Troposphere Research (IMKTRO) of the Karlsruhe Institute of Technology (KIT), and further consists of the University of Mainz, Ludwig-Maximilians-University of Munich and DLR’s (German Aerospace Center) Institute of Atmospheric Physics. At an international level, NAWDIC has matured into a large pan-Atlantic consortium with scientific partners from 10 countries involving universities, research institutions and weather services. NAWDIC is endorsed by WMO’s World Weather Research Programme (WWRP).

Despite significant advancements of state-of-the-art NWP models in recent decades, accurately forecasting the location, timing, and intensity of mesoscale HIW events remains a challenge. This is to a large degree due to the cross-scale interactions of physical processes involved in the formation of HIW. In midlatitudes, the processes range from upper-tropospheric Rossby waves covering thousands of kilometers and lasting several days to turbulent momentum transport in the planetary boundary layer (PBL) and cloud microphysical processes acting on scales of hundreds of meters to micrometers and minutes to seconds. A cross-scale airstream that connects upper-tropospheric Rossby waves over North America and the Atlantic Ocean with HIW in Europe is the dry intrusion (DI). During winter months, DIs emerge most frequently from the downstream flank of upper-tropospheric ridges over eastern North America. From this region (referred to as ‘DI inflow’), the DI descends equatorward into the cold sector of a downstream extratropical cyclone over a horizontal distance of 1000-5000 km and reaches the PBL about 2 days later (referred to as ‘DI outflow’). The DI outflow is accompanied by intense surface heat and moisture fluxes, elevated PBL heights, changes in PBL cloud cover, and a destabilization of the lower troposphere leading to unusually strong wind gusts and extreme rainfall due to deep convection. Most of the time, the involved cross-scale interactions of physical processes relevant to HIW in Europe occur upstream over the Atlantic Ocean and are insufficiently captured by operational observing systems.